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Why is default interpolation done this way?

My question (before I give the backstory):
Why does the specification subtract 0.5 from coordinates when interpolating? The values I wish to use are stored at the vertices of my texture. This seems to be a "duh!" thing. I need values interpolated between these values. When I add 0.5 back into my coordinates, my output is very close to what I expect it to be, as you can see in my test output below.

My Texture Sampler:
- NORMALIZED_FALSE
- ADDRESS_NONE
- LINEAR

On page 213 of the Specification rev 1.0.48 describes exactly what I am questioning in this excerpt:

Re: Why is default interpolation done this way?

Originally Posted by david.garcia

[1] Spec lawyers, please don't shoot.

Thats what I figured. Someone has to draw the line between CL and GL somewhere and I can find no good arguments for or against changing it from how OpenGL does it. At least, none that I can't immediately counter again.